1. Field of the Invention
Example embodiments relate to an amplifier and method of reproducing sound. In particular, example embodiments relate to a digital amplifier and a method of reproducing sound in which a reference voltage related to an analog DC component of an output node is provided to a reference node.
2. Description of the Related Art
Various conventional amplifiers have been used to amplify audio signals. These conventional amplifiers include class-A, class-B, class-AB and class-D amplifiers. Generally, class-D amplifiers have superior power efficiency characteristics as compared with the class-A, class-B and class-AB amplifiers.
Class-D amplifiers are commonly used in portable devices at least in part because of an increased emphasis that is generally placed on the size and weight of portable devices. For example, headphones associated with various portable audio players include class-D amplifiers.
FIG. 1 is a basic block diagram illustrating a conventional class-D amplifier 100, and FIGS. 2A-E illustrate a simplified example of the signal processing performed by each of the components of the class-D amplifier 100. The operation of the conventional class-D amplifier 100 is explained below referring to FIGS. 1 and 2A-E.
The class-D amplifier 100 includes a Pulse Width Modulation (PWM) signal generator 10, a class-D driving circuit 20, a low pass filter 30 and a coupling capacitor C2. FIG. 2A illustrates the PWM signal generator 10 receives a first signal 13 and a second signal 11. The first signal 13 is shown in FIG. 2A as a square wave. For example, the first signal 13 may be received from an internal or external clock. The second signal 11 received by the class-D amplifier 100 is referred to hereinafter as an audio input signal 11. In order to simplify the explanation, the received audio input signal 11 is illustrated in FIG. 2A as a sinusoidal wave. The PWM signal generator 10 processes the first signal 13 and the audio signal 11 and outputs a PWM signal 15 to the class-D driving circuit 20. The PWM signal 15 is shown in FIG. 2B, and the duty cycle of the PWM signal 15 is varied according to the received audio signal 11. The class-D driving circuit 20 amplifies the PWM signal 15 and provides the amplified signal 25 shown in FIG. 2C to the low pass filter 30. The low pass filter 30 averages the amplified signal 25, thereby reducing high frequency noise, and provides the filtered signal 31 to the coupling capacitor C2. The coupling capacitor C2 removes the DC voltage from the filtered signal 31 and outputs an output signal 33. In FIG. 1, the output signal 33 is provided to a speaker 150, which has a resistance RL that is connected to the class-D amplifier 100. The speaker 150 is a speaker, for example, included in headphones for a portable audio device.
As described above, a conventional class-D amplifier 100 includes the coupling capacitor C2 for removing the DC voltage from the filtered signal 31. Further, the coupling capacitor C2 is used prevent high currents from flowing through the headphones and having the headphones be in a continuously on state. If the resistance RL of the speaker 150 is about 16-32 ohms, which is typical, the capacitance value of the coupling capacitor C2 is typically within a range of 100-470 μF. However, the physical size of a 100-470 μF coupling capacitor is prohibitively large and thus, inhibits the miniaturization of the conventional class-D amplifier 100 including one or more coupling capacitors.
As such, other conventional amplifiers have been developed that do not use and/or require the coupling capacitor. FIG. 3 is a block diagram of a prior art amplifier 200 disclosed in International Publication No. WO 2006/031304, which does not include a coupling capacitor.
Referring to FIG. 3, the prior art amplifier 200 includes a first amplifier 21 driving a speaker of a left headphone, a second amplifier 22 driving a speaker of a right headphone, and a DC voltage-to-voltage converter 40. The first amplifier 21 is connected to a headphone load RL via a connecting lead 51, and the second amplifier 22 is connected to a headphone load RL via a connecting lead 52. Each of the first amplifier 21, the second amplifier 22, and the DC voltage-to-voltage converter 40 receive a supply voltage VDD. The DC voltage-to-voltage converter 40 is a “charge pump” and converter uses charge pump circuitry including capacitors or inductors to store and transfer energy. The DC voltage-to-voltage converter 40 is used instead of coupling capacitors, which would be located in series between the output of the first amplifier 21 and the speaker of the left headphone and the output of the second amplifier 22 and the speaker of the right headphone.
The charge pump circuitry of the prior art amplifier shown in FIG. 3 generates a negative voltage rail −VDD with respect to ground. The negative voltage rail −VDD provided by the charge pump circuitry powers the first amplifier 21 and the second amplifier 22, and drives amplifier operation from both positive and negative rails. Providing a negative voltage rail −VDD with respect to ground allows for the headphone amplifiers to be biased at a ground voltage, thereby allowing the incoming signals to be amplified without clipping.
As such, the prior art headphone amplifier circuit 200 allows for the headphone speakers represented by RL to be biased at zero volts, operating between VDD and −VDD, which in turn allows for the lead 51 of the first amplifier 21 and the lead 52 of the second amplifier 22 to be directly coupled to the headphone speakers RL, without the need for any DC coupling capacitors.
Because the DC voltage-to-voltage converter 40 is a capacitive or inductive charge pump, meaning the charge pump circuitry includes capacitors or inductors to store and transfer energy, charge and discharge are needed to artificially generate the negative voltage rail −VDD from the supply voltage VDD. The charge and discharge operations of the DC voltage-to-voltage converter 40 significantly increase power consumption of the headphone amplifier circuit 200 as compared to a conventional amplifying circuit including the coupling capacitors, such as the conventional class-D amplifier 100.